Limited understanding of basic ocean processes is hindering progress in marine carbon dioxide removal

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To limit warming to <2 °C, we need both emissions reductions and carbon dioxide removal (CDR). A diverse range of potential CDR methods have been proposed to achieve billion-tonne (i.e. gigatonne, Gt) annual CO₂ removal rates within 30–50 years, with multiple approaches needed to be developed and upscaled massively to achieve that goal. The need for robust criteria to assess the viability of candidate CDR mechanisms has long been recognised, yet new methods are being proposed regularly with insufficient exploration of such checks or balances. This is particularly true for ocean-based CDR, now attracting greater interest as the constraints on land-based methods become apparent.